Isoolefin polymers are prepared in carbocationic polymerization processes. Of special importance is butyl rubber which is a copolymer of isobutylene with a small amount of isoprene. Butyl rubber is made by low temperature cationic polymerization that generally requires that the isobutylene have a purity of >99.5 wt % and the isoprene have a purity of >98.0 wt % to prepare high molecular weight butyl rubber.
The carbocationic polymerization of isobutylene and its copolymerization with comonomers like isoprene is mechanistically complex. See, e.g., Organic Chemistry, SIXTH EDITION, Morrison and Boyd, Prentice-Hall, 1084-1085, Englewood Cliffs, N.J. 1992, and K. Matyjaszewski, ed, Cationic Polymerizations, Marcel Dekker, Inc., New York, 1996. The catalyst is typically composed of two components: an initiator and a Lewis acid. Examples of Lewis acids include AlCl3 and BF3. Examples of initiators include Brønsted acids such as HCl, RCOOH (wherein R is an alkyl group), and H2O. During the polymerization process, in what is generally referred to as the initiation step, isobutylene reacts with the Lewis acid/initiator pair to produce a carbenium ion. Following, additional monomer units add to the formed carbenium ion in what is generally called the propagation step. These steps typically take place in a diluent or solvent. Temperature, diluent polarity, and counterions affect the chemistry of propagation.
Industry has generally accepted widespread use of a slurry polymerization process (to produce butyl rubber, polyisobutylene, etc.) in the diluent methyl chloride. Typically, the polymerization process extensively uses methyl chloride at low temperatures, generally lower than −90° C., as the diluent for the reaction mixture. Methyl chloride is employed for a variety of reasons, including that it dissolves the monomers and aluminum chloride catalyst but not the polymer product. Methyl chloride also has suitable freezing and boiling points to permit, respectively, low temperature polymerization and effective separation from the polymer and unreacted monomers. The slurry polymerization process in methyl chloride offers a number of additional advantages in that a polymer concentration of approximately 26% to 37% by volume in the reaction mixture can be achieved, as opposed to the concentration of only about 8% to 12% in solution polymerization. An acceptable relatively low viscosity of the polymerization mass is obtained enabling the heat of polymerization to be removed more effectively by surface heat exchange. Slurry polymerization processes in methyl chloride are used in the production of high molecular weight polyisobutylene and isobutylene-isoprene butyl rubber polymers, polymerization of isobutylene and para-methylstyrene, and are star-branched butyl rubber.
However, there are a number of problems associated with the polymerization in methyl chloride. For example, there can be a tendency of the polymer particles in the reactor to agglomerate with each other and to collect on the reactor wall, heat transfer surfaces, impeller(s), and the agitator(s)/pump(s). The rate of agglomeration increases rapidly as reaction temperature rises. Agglomerated particles tend to adhere to and grow and plate-out on all surfaces they contact, such as reactor discharge lines, as well as any heat transfer equipment being used to remove the exothermic heat of polymerization, which is critical since low temperature reaction conditions must be maintained.
Another problem associated with slurry polymerization is the tendency of the polymer to form or deposit on the reactor surfaces. This manner of polymer formation or deposition occurs when the polymer forms directly on the reactor surfaces, and is referred to herein as “film deposition” or “deposition” to distinguish from the agglomeration and collection of “sticky” polymer particles from the slurry, which is referred to herein as “polymer agglomeration,” “particle agglomeration” or “agglomeration.” The rate of polymer film deposition on the reactor surfaces is generally proportional to the rate of polymerization, whereas particle agglomeration depends more on the characteristics of the slurry, flow conditions, particle adhesion, etc.
The commercial reactors typically used to make these rubbers are well mixed vessels of greater than 10 to 30 liters in volume with a high circulation rate provided by a pump impeller. The polymerization and the pump both generate heat and, in order to keep the slurry cold, the reaction system needs to have the ability to remove the heat. An example of such a continuous flow stirred tank reactor (“CFSTR”) is found in U.S. Pat. No. 5,417,930, incorporated by reference, hereinafter referred to in general as a “reactor” or “butyl reactor.”
In butyl reactors, slurry is circulated through tubes of a heat exchanger by a pump, while boiling ethylene on the shell side provides cooling, the slurry temperature being determined by the boiling ethylene temperature, the required heat flux and the overall resistance to heat transfer. On the slurry side, the heat exchanger surfaces progressively accumulate polymer, either by agglomeration or deposition, inhibiting heat transfer, which would tend to cause the slurry temperature to rise. The resistance to heat transfer can be monitored by observing inlet and outlet temperature differences and the flow rate of the coolant, and taken as an indication of the extent of polymer accumulation. When the heat transfer resistance from polymer accumulation on the heat transfer surfaces becomes excessive, the reactor is taken out of service for cleaning. The subject of polymer accumulation has been addressed in several patents (such as U.S. Pat. No. 2,534,698, U.S. Pat. No. 2,548,415, U.S. Pat. No. 2,644,809). However, these patents have unsatisfactorily addressed the myriad of problems associated with film deposition on heat transfer surfaces for implementing a desired commercial process.
Chinese Patent Application No. 01134710.4, Public Disclosure No. CN 1417234A, discloses a method for the preparation of isoolefin polymers or copolymers by cationic polymerization in which a homopolymerization reaction of C4-C7 isoolefin monomers or a copolymerization reaction with other monomers is performed in a chlorohydrocarbon diluent using a Lewis acid as the primer, to which reaction system it is suggested to add such dispersing agents as carboxylic acid esters, ethers, ketones, amines, styrenes or alkyl substituted styrenes. The dispersing aids are said to lower the viscosity of the polymerization system and to make the dispersion of the insoluble polymer granules more uniform in the diluent. The reference claims that at a reaction temperature below −20° C., a stably dispersed polymer system can be obtained, the problem of heat transfer and mass transfer can be effectively improved, the dispersing agent that has been added can be easily obtained, and, at the same time, a narrower molecular weight distribution (MWD) of the polymer is obtained. However, there is no disclosure of any specific co-initiator for the Lewis acid, and some of the alleged dispersing aids are known comonomers.
Hydrofluorocarbons (HFC's) are chemicals that are currently used as environmentally friendly refrigerants because they have a very low (even zero) ozone depletion potential. Their low ozone depletion potential is thought to be related to the lack of chlorine. The HFC's also typically have low flammability particularly as compared to hydrocarbons and chlorinated hydrocarbons. HFC's have recently been disclosed as a polymerization system diluent. Some polymerization media, processes, reactors and systems that can employ HFC's are disclosed in the following commonly assigned patent references: WO2004058827; WO2004058828; WO2004058829; WO2004067577; WO2006011868; US2005101751; US2005107536; US2006079655; US2006084770; US2006094847; US2006100398; and US2006111522.
WO 02/34794 discloses a free radical polymerization process using hydrofluorocarbons. Other background references include DE 100 61 727 A, WO 02/096964, WO 00/04061, U.S. Pat. No. 5,624,878, U.S. Pat. No. 5,527,870, and U.S. Pat. No. 3,470,143.
Finding alternative blends of diluents, or a diluent additive to improve polymerization methods and systems that would reduce film deposition on reactor surfaces is desirable. Such improved methods and systems would reduce polymer deposition and reactor heat transfer resistance without compromising process parameters, conditions, or components and/or without sacrificing productivity/throughput and/or the ability to produce high molecular weight polymers.